If they didn't receive your money until after betting was closed, then you didn't purchase anything. Whatever is written in the rules can't take away your consumer rights. If they don't provide the service paid for or send a refund it's just theft. There not protecting anyone by keeping the money, just lining there pockets.
The service is GAMBLING. GAMBLING is risky. When bets are time average weighted you
must discourage late bets. Why?
Masses of idiots like you would spam the blockchain at the last possible second when the bet outcome is known, for even a 1% winnings. This screws early bettors and creates 95% of the work for bitbet admins for 5% of the customers. The same 5% of childish people that bitbet is discouraging, because they are the worst kind of customers.
This OPEN discouragement of late betting is clearly explained as such, in the faq.
You're telling me you went and sent 10btc, at the time, ~$8,000, without even reading the faq? And you're blaming others for this oversight?
Gambling is risky but the risk is supposed to be in losing the bets and not getting screwed by the site operator. Having a rule that payments must be confirmed before a bet is resolved to count is perfectly reasonable. Having a rule that refunds of such payments are subject to a small convenience fee (since it may require manual intervention to refund) is also perfectly reasonable. Having a rule that says bets don't count up to X minutes before a bet is resolved is also reasonable.
A price of almost $750 is not $750 though, so unless the bet was placed after the price actually crossed $750 then I don't think it's an exploit - the outcome was still in doubt even if the doubt was very small. Regardless, keeping 100% of the money is stealing, and a wave of people from BitBet IRC chat saying otherwise doesn't change that.
A FAQ is for when people have questions, it's not a must-read for casual users who want to make sure they don't get screwed by awful policies. Even actual must-reads like terms and conditions you must agree to before using a service do not give you the right to steal.
OP, I hope you contact a lawyer if the site continues to refuse to refund you. I bet a judge would see this as stealing, but running this by a lawyer would be the best way to be sure. You're out a significant chunk of change so I believe it could easily be worth it, and you'd be doing the bitcoin community a service.